Humans didn’t evolve from modern monkeys, but if you trace the branches of our family tree far enough, you’ll realize that we share a common ancestor. Here’s what they left us with.

1. Coccyx, Our Former Tail

Why does falling on your tush hurt so much? Because the tailbone is a remnant of your long-lost tail. (For about four weeks, human embryos have a tail. In rare cases, people are born with them!) The tail disappeared millions of years ago when hominids started walking upright and no longer needed it for balance. However, its absence has left the bottom of our spinal columns exposed. That’s why your coccyx is so easy to bruise and break.

2. Our Complex Hands

Primates are the only mammals with opposable thumbs. Notharctus, a lemur-like monkey that lived 50 million years ago, was the first ape to develop human-like hands: A thumb, long fingers, and nails instead of claws. Why? They were—and still are—perfect for clinging to tree branches!

3. The Ability to See Colors

For millions of years, our ancestors were red-green colorblind. But thanks to receptors called “opsins,” everything went technicolor around 23 million years ago. Most colorblind animals have two sets of opsin genes. Humans, however, have three—and that third gene makes all the difference. Scientists posit that millennia ago, an opsin gene duplicated and mutated and was a huge advantage. (Scientists have confirmed this hypothesis by planting a third opsin gene in the retina of colorblind squirrel monkeys. The experiment gave them human-like color-vision.)

4. Our Crummy Sense of Smell

You can’t have it all. As our sense of vision got better, our sense of smell got worse. We have thousands of genes for smell, but nearly 600 of them don’t work anymore.

5. The Ability to Take a Stroll

For about 365 million years, most animals walked on all fours. But around 4.4 million years ago, a woodland primate called ardipithecus stood up and walked with an awkward wobble. After a couple million years, australopithecus emerged (its most famous member is “Lucy”). Just like humans', its knees bent inward, making walking more natural.

6. Our Bad Backs

Walking came with a cost: it ruined our backs. In order to stay balanced, our ancestors developed an “S-shaped” spine, which—as we all know—leads to kinks, knots, sciatica, and all sorts of pain. But all that aching might be worth it. Standing freed up our hands and gave us a chance to make tools.